There is an increasing demand for higher capacities for lithium ion secondary batteries, due to the performance improvement of mobile devices such as a smart phone, a tablet device and the like, and popularity of vehicles equipped with lithium ion secondary batteries such as EV, PHEV and the like. Currently, graphite is primarily used as an anode material in a lithium ion secondary battery. However, to achieve a higher capacity, an anode material that uses metals such as silicon or tin, or other elements, which have higher theoretical capacities and is capable of storing and releasing lithium ions is under active development.
On the other hand, such an active material which contains a metal material capable of storing and releasing lithium ions is known to exhibit a significant volume increase upon alloying with lithium by charging. Due to this volume increase, the active material breaks into finer pieces and an anode in which the material is used also breaks up in its structure resulting in the loss of conductivity. Therefore, the decrease in the capacity after many cycles is a problem for the anode using such a metal material.
To solve this problem, a method of making these metal materials into fine particles and making a composite of these fine particles and carbonaceous materials or graphite is proposed. In these composite particles, these metal materials create an alloy with lithium, resulting in retention of conductivity due to carbonaceous materials or graphite even when the active material breaks into finer pieces. Thus, these composite particles are known to exhibit significant improvement of cycle characteristic compared to the material in which such a material is used alone as an anode material. For example, Patent Document 1 discloses an anode active material including a fine particle on which a carbonaceous material is formed, and said fine particle contains at least one kind of element selected from the group consisting of Mg, Al, Si, Ca, Sn and Pb, has an average particle size of 1 to 500 nm and an atomic ratio of the fine particle in the active material is not lower than 15 wt %.
Patent Document 2 discloses a metal-carbon composite particle, in which a metal particle is buried in a plurality of phases of carbon that contains graphite and amorphous carbon. It is described that the metal particle is composed of any one of Mg, Al, Si, Zn, Ge, Bi, In, Pd, or Pt, and the average particle size of the metal particles is preferably 0.1 to 20 μm. Patent Document 3 discloses an anode active material that has a so-called core-shell structure, which includes graphite core particles and carbon layer (shell) covering said graphite core particle and metal particles disposed in said carbon layer as dispersed inside said carbon layer. Preferably, the average particle size of the graphite core particles is 1 to 20 μm, the coating thickness of the carbon layer is 1 to 4 μm, metal that alloys with the lithium contains at least one material selected from the group consisting of Cr, Sn, Si, Al, Mn, Ni, Zn, Co, In, Cd, Bi, Pb, and V and the average particle size thereof is 0.01 to 1.0 μm.
Furthermore, Patent Document 4 describes a method of manufacturing a composite active material for a lithium secondary battery, which includes the steps of mixing and of conglobation: in the step of mixing, expanded graphite or flake graphite with a BET specific surface area of 30 m2/g or greater and a battery active material capable of compounding with lithium ions are mixed to obtain a mixture; in the step of conglobation, the conglobation treatment is applied to said mixture to manufacture a substantially spherical composite material for a lithium secondary battery including a battery active material capable of compounding with graphite and lithium ions. The battery active material capable of compounding with lithium ions preferably contains at least one type of elements selected from the group consisting of Si, Sn, Al, Sb, and In, and the average particle size thereof is preferably not greater than 1 μm.
The use of these fine metal materials can reduce the expansion per particle caused by insertion of lithium during charging and reduce breakage of the material, thus improving the cycle life. However, the performance does not satisfy the requirement yet, and further improvement of the cycle life is needed.
In the method described above, in which the composite particles are used, the denser the composite particles fills an anode thin film, the higher the energy density of the anode, thereby improving the battery performance. Also, by packing the composite particles uniformly and as isotropic as possible, lithium enters and exits more uniformly and topical degradation of the anode can be avoided, resulting in the improved cycle life. For example, Patent Document 5 discloses an anode material for a lithium secondary battery, including a spherical graphite particle originating in a scaly natural graphite particle, and the circularity thereof is preferably not less than 0.85.